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Abstract
The overhead transparency system has evolved to a specification different from that for the 35 mm trallsparency. 11 was found that the background illumination present in the former situation is the reason for the difference. A Dmax higher than 1.5 does not result in an observable screen change. Also screen luminances are much higher than those seen for 35 mm systems. These observations were experimentally verified, simulated by a computer model, and then compared with a range of commercially-generated overhead transparellcies.